582 



russell-dickey. POROSITY AND PERMEABILITY 



[Ch. 32 



the oil saturation and reduces the effective permeability of the sand 

 to oil still further. It also increases the viscosity of the oil and con- 

 sequently decreases its flow. 



The great majority of oil pools have been produced by this depletion 

 mechanism (Mullane, 1944), which is known as "dissolved-gas drive." 



1936 1937 1938 1939 1940 

 Fig. 1. Graphical statistics of Northwest Gloyd Extension of the Rodessa Field. 

 (After Mullane, 1944, p. 54.) 



The graph of production against time follows a very definite pattern. 

 The oil flow reaches its maximum during the first few weeks, and after 

 a few months it is declining rapidly. The decline continues, but the 

 rate of decline diminishes, and, if the logarithm of the production is 

 plotted against the logarithm of time, the production-decline curve is 

 approximately a straight line. Meanwhile the gas-oil ratio, usually 

 expressed as cubic feet per barrel, increases rapidly from that originally 

 dissolved in the oil to a high maximum and then falls as the gas sup- 

 ply is exhausted. Figure 1 shows the oil production and gas-oil ratio 



